IMO anyone who rags on the banjo has never heard a REALLY good banjo player do his stuff.
If you want to see some excellent banjo pickin' do a Google search for "Roy Clark banjo" and watch a few YouTube videos.
The banjo is a modern instrument with roots in African music. It was imported to America via the slave trade, and over time the original 4-string banjo was modified to include a tambourine/drumhead style body, and a fifth string. Those two features of the modern banjo make it one of the very few uniquely American instruments.